1. Technical Field
The present invention relates to a pressure sensor that does not use oil as a pressure receiving medium and therefore is suitable for broadening a scope of application thereof.
2. Related Art
Pressure sensors that use a piezoelectric resonator as a pressure sensitive element are known as a water pressure gauge, an air gauge, and a differential pressure gauge. The piezoelectric resonator includes, for instance, an electrode pattern on a planar piezoelectric substrate, and a direction in which a force is detected set to be a detecting axis. When pressure is applied in the direction of the detecting axis, a resonance frequency of the piezoelectric resonator changes and the pressure is detected by using the fluctuation in the resonance frequency. JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534, as a first, second, and third examples, disclose a pressure sensor including a piezoelectric resonator as a pressure sensitive element. When a pressure is applied to bellows from a pressure input orifice, a force F corresponding to an effective area of the bellows is transmitted to the piezoelectric resonator as a compressive force or a tensile force through a force transmitting unit that has a pivot as a fulcrum (a flexible hinge). A stress corresponding to the force F appears in the piezoelectric resonator and this stress changes the resonance frequency. The pressure sensor measures pressure by detecting a change in the resonance frequency appearing in the piezoelectric resonator.
A pressure sensor in related art will be described with reference the first example and the like. FIG. 11 is a schematic view showing a structure of a pressure sensor in related art.
A pressure sensor 101 according to related art shown in FIG. 11 includes a case 104 having first and second pressure input orifices 102 and 103 that are arranged to face each other, and a force transmitting member 105 disposed inside the case 104. A first end of the force transmitting member 105 is sandwiched with and coupled to one end of a first bellows 106 and one end of a second bellows 107. The other end of the first bellows 106 is coupled to the first pressure input orifice 102, and the other end of the second bellows 107 is coupled to the second pressure input orifice 103. Moreover, a double-ended tuning fork resonator 109 serving as a pressure sensitive element is disposed between a second end of the force transmitting member 105 and an end portion of a substrate 108 which is an opposite end from a pivot (fulcrum).
Here, the bellows of this pressure sensor is filled with a liquid so as to detect pressure with high precision. Generally, oils such as silicon oil which has high viscosity are commonly used as the liquid, in order to prevent bubbles from entering and accumulating inside the bellows or between the folds of the bellows.
Thus, the interior of the first bellows 106 is filled with oil 110 having viscosity. In a case where an object for a pressure measurement is a liquid, the oil 110 faces to be brought into contact with the liquid at an opening 111 opened at the first pressure input orifice 102. Here, a size of the opening 111 is set such that the oil 110 does not leak out.
In the pressure sensor 101 having such structure, when the pressure F is applied to the oil 110, which fills the first bellows 106, from the liquid that is an object for pressure measurement, this pressure F is then applied to the first end of the force transmitting member 105 (a pivotably supported swing arm) through the first bellows 106. At the same time, atmospheric pressure is applied to the second bellows 107 and a force equivalent to the atmospheric pressure is applied to the first end of the force transmitting member 105.
Consequently, a force equivalent to a differential pressure is applied through the second end of the force transmitting member 105 to the double-ended tuning fork resonator 109 as a compressive force or a tensile force with a pivot of the substrate 108 as a pivoting point. The differential pressure means a pressure difference between the atmospheric pressure and the pressure F applied by the liquid that is the object for pressure measurement. Due to the compressive force or the tensile force applied to the double-ended tuning fork resonator 109, a stress is generated in the resonator 109. In accordance with a strength of the stress, the resonance frequency of the resonator 109 changes. Therefore, measurement of the resonance frequency enables detection of the strength of the pressure F.
JP-A-2005-121628, as a fourth example, discloses a sensor having such structure that does not include an expensive force transmitting unit (cantilever), which is used in the pressure sensor of the above examples, having a swing arm using a pivot (the flexible hinge) as a fulcrum. In the sensor, two bellows are directly aligned in a sensor housing in a manner sandwiching a pedestal therebetween. The sensor detects a pressure fluctuation generated by a behavior of the pedestal attributing to the difference between pressures lead into each of the bellows. Therefore, a resonator bonding pedestal is sandwiched between one end of the first bellows and one end of the second bellows. A pressure sensitive element is provided at a circumference side of the second bellows and ends of the pressure sensitive element are fixed on the pedestal and on a housing wall positioned at the other end side of the second bellows. Further, a reinforcing board is disposed at an axisymmetrical position to the pressure sensitive element with the second bellows interposed. The ends of the reinforcing board are fixed at the pedestal and at the housing wall.
JP-A-2007-57395 as a fifth example discloses a pressure sensor including a reinforcing flexible member (that is, a string) that connects a pedestal and a housing and are disposed in a direction orthogonal to a direction of a pressure detecting axis. The reinforcing flexible member is provided so as to solve such problem that the sensor disclosed in the fourth example has insufficient strength with respect to a shock coming from a direction orthogonal to a direction of a pressure detecting axis of the bellows.
JP-A-2006-194736 and JP-A-2007-132697 as sixth and seventh examples disclose a pressure sensor that is used in a fixed manner to an engine block so as to detect hydraulic pressure inside an engine. This pressure sensor includes: a sensing unit that outputs an electric signal corresponding to an applied pressure; a pressure-receiving diaphragm unit that receives pressure; and a pressure transmitting member for transmitting the pressure from the diaphragm unit to the sensing unit. Specifically, a first diaphragm for receiving pressure and a second diaphragm for detection are respectively installed on each end surface of a hollow metal stem. The pressure transmitting member is provided between the first diaphragm and the second diaphragm in the stem. The pressure transmitting member is a shaft made of metal or ceramic, and is provided between the pair of diaphragms in a prestressed state. Further, a chip with a strain gauge functionality is installed to an outer end surface of the second diaphragm as a pressure detection element. The pressure transmitting member transmits pressure received by the first diaphragm to the second diaphragm, and deformation of the second diaphragm is converted into an electronic signal by the strain gauge chip, thereby detecting the hydraulic pressure of the engine.
In the first to third examples, the first bellows 106 is filled with the oil 110 as the pressure sensor shown in FIG. 11. The oil 110 has a high thermal expansion coefficient compared to other elements that constitute the pressure sensor 101, such as the force transmitting member 105 and the double-ended tuning fork resonator 109. Therefore, thermal distortion occurs in the components constituting the pressure sensor due to a temperature change. Such thermal distortion works on the double-ended tuning fork resonator 109 as unwanted stress, resulting in an error of a measured pressure value. Thus, the characteristics of the pressure sensor are degraded.
Moreover, since the oil 110 filling the first bellows 106 contacts and faces a liquid that is an object for pressure measurement, the oil 110 may flow into the liquid, or, the liquid may flow into the first bellows 106 depending on how the pressure sensor is installed. This may generate bubbles inside the oil 110 filling the first bellows 106. If bubbles are generated in the oil 110 that serves as a pressure transmitting medium, a force cannot be stably transmitted through the force transmitting member 105 to the double-ended tuning fork resonator 109, thereby possibly inducing an error in a measured pressure value.
Further, as described above, since the oil 110 contacts and faces the liquid that is an object for pressure measurement, the oil 110 may flow into the liquid depending on how the pressure sensor is installed. Therefore, the pressure sensor using the oil 110 according to related art is not applicable to measurement of pressure of a pure liquid that dislikes foreign substances.
Furthermore, the pressure sensor 101 of related art includes the force transmitting member 105 having a complicated structure, thereby being difficult to be miniaturized. In addition, the force transmitting member 105 requires a flexible hinge having a slim constriction so as to be an expensive component, thereby disadvantageously increasing a cost for manufacturing a pressure sensor.
When the pressure sensor of the fourth and fifth examples inclines, the bellows thereof droops. Therefore, a force applied to the pressure sensitive element (the double-ended tuning fork resonator) varies, resulting in a change of a resonance frequency.
Further, the pressure sensor of the fourth and fifth examples has such structure that one end of a pipe filled with an oil is connected to a pressure introduction orifice of the pressure sensor and the other end of the pipe is brought into contact with a liquid that is a measurement object. Therefore, as is the case with the first to third examples, the oil filling the bellows or the pipe contacts and faces the liquid that is the object for the pressure measurement. Accordingly, the oil may flow into the liquid that is the pressure measurement object depending on how the pressure sensor is installed, or bubbles may be generated in the oil filling the bellows. If bubbles are generated in the oil, the oil can not stably transmit a force through the pedestal to the double-ended tuning fork resonator, resulting in an error of the pressure measurement.
The pressure sensor of the fifth example has such structure that the pedestal sandwiched by the bellows is supported by the reinforcing flexible member that is a plate string provided at the lateral surface of the housing. Therefore, a force suppressing a behavior, corresponding to a move of the bellows in the axis direction, of the pedestal acting in the sensor. Therefore, pressure detection sensitivity may be deteriorated. If the reinforcing flexible member is hardened for its firm support, the move of the bellows is suppressed, deteriorating the pressure detecting sensitivity.
Further, in the fourth and fifth examples, the reinforcing board is disposed at an axisymmetrical position to the pressure sensitive element with the bellows interposed. Therefore, a move is the bellows is suppressed, deteriorating the pressure detecting sensitivity.
In the sixth and seventh examples, the diaphragm and the shaft are in contact with each other in the prestressed state. The pressure sensor is used at a high temperature in a high pressure. Therefore, if the diaphragm and the shaft were rigidly fixed, the mechanism may be damaged by the difference between thermal expansions of the components. In consideration of the thermal expansions, the diaphragm and the shaft only have a point contact, and are not bonded by bonding means such as adhesives. Therefore, there is a very high possibility that this point contact deviates when the diaphragm and the shaft operate by the pressure change. As the point contact deviates, a force working in both of the diaphragm and the shaft leaks out, resulting in pressure detection with poor precision. Moreover, the pressure sensor of the sixth and seventh examples is used at a high temperature in a high pressure. Therefore, it is desirable that the force transmitting member be as long as possible in order to create a distance between the pressure receiving unit and the sensing unit and avoid thermal effect on the components such as the chip of the sensing unit. Thus the sensor of the examples is not suitable for miniaturization. In addition, in the case of the sixth and seventh examples, a force is transmitted with a shaft disposed between a pair of diaphragms. However, since a sensor chip is attached to the diaphragm on the sensing unit, the property of the diaphragms differs between the pressure receiving side and the sensing unit side. Therefore, the measurement accuracy can not be improved.